Fischer-Tropsch carbon monoxide hydrogenation is a catalysed process in which a carbon monoxide and hydrogen mixture, typically referred to a “synthesis gas” or “syngas”, is converted into liquid hydrocarbons, predominantly linear hydrocarbons (olefins and paraffins) of different chain length. Small amounts of oxygenates, mainly alcohols, aldehydes and acids can also be formed. The products can be used to produce transportation fuels such as diesel and petrol as well as waxes. Alternately, or in addition, valuable chemicals such as olefins and oxygenates can be extracted from the product.
The synthesis gas can include carbon dioxide instead of carbon monoxide and water instead of hydrogen, and may alternatively include combinations of these reactants.
Literature from the 1960's and 1970's suggests that linear primary amines may be formed upon co-feeding of ammonia and other nitrogen-containing compounds to the synthesis gas during Fischer-Tropsch synthesis or other reactions involving synthesis gas. This may be a result of using iron based catalysts prevalent at the time as iron is known to catalyse ammonia production. Thus Kölbel H., Abdulahad I., Ralek M. Erdöl Kohle 28 (1975) 385 showed that the addition of ammonia or amines to Fischer-Tropsch synthesis gas results in the formation of higher alkyl substituted amines.
DE904891 describes adding of ammonia (1.3 Vol. %) to Fischer-Tropsch synthesis gas (CO:H2=1:1) and reacting over a copper or alkali promoted fused iron catalyst to yield up to 18 wt. % nitrogen containing compounds in the liquid product, primarily in the form of amines.
U.S. Pat. No. 2,821,537 describes the addition of ammonia or methylamine during Fischer-Tropsch synthesis over potassium promoted precipitated iron catalyst. Addition of up to 2 vol % ammonia at 30 bar and 190 to 210° C. led to formation of 10-20 wt % nitrogen containing compounds with primary linear amines making up the bulk of these.
U.S. Pat. No. 3,726,926 discloses the formation of linear alkylamines of chain length 3-22 with a selectivity of 20 to 40 wt % during Fischer-Tropsch synthesis with ammonia addition to H2 and CO over group VIII metal, supplemented by smaller amounts of group III and group IA and IIA metals at conditions ranging from 160-220° C. and 50-200 bar. Typical feed composition ratios of NH3:CO:H2 were 0.03-0.5:0.8-1.2:1-3.
U.S. Pat. No. 4,272,452 discloses a process for the preparation of acetonitrile (CH3CN), a very short chain nitrile, from CO, H2 and ammonia at high temperatures (350-600° C.) over transition metals such as molybdenum and iron. These are not Fischer-Tropsch reactions conditions.
Kölbel, H., and Ralek M 1984, ‘The Kölbel-Engelhardt Synthesis’ in R. B. Anderson (ed), The Fischer-Tropsch Synthesis, Academic Press, pp. 287-288 discloses the formation of primary aliphatic amines (C1-C20) to up to 25 wt. % of total product from a CO, H2O and ammonia feed. It is further reported that higher partial pressure of ammonia results in shorter hydrocarbon chains of the amines formed. Typical catalysts used were precipitated iron bulk catalyst promoted with 0.2 wt. % Cu and 0.6 wt. % K. Typical reaction conditions were 11 bar and 219-235° C. No formation of nitriles or amides is reported in this document and the use of a slurry reactor as opposed to a fixed-bed reactor was reported to be unfavourable.
More recently, cobalt based catalysts have become an alternative to iron based catalysts as the most widely used in Fischer-Tropsch reactions and studies have found that nitrogen and nitrogen containing products act as a poison to these catalysts. Thus, whilst it is highly desirable to produce linear nitriles, amides and formamides from a synthesis gas feed, the prior art indicates that this is not viable as only amines are produced and as nitrogen based compounds poison cobalt based catalysts.
In this specification, linear phosphorous containing compounds shall mean organic compounds which include a phosphorous atom.